Update index.js for node.js 8+

[nbards]

Changes per https://cloud.google.com/functions/docs/migrating/nodejs-runtimes

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[fhinkel]

@ace-n Could you have a look. Thanks

[ace-n]

@nbards can you do the following?

1. Sign the CLA
2. Update the comments above the method (to match the new parameters)

Once those are done, I can review this in more detail. Thanks!

[sdenef-adeo]

Maybe you can close this PR as it is resolved by #1335

[ace-n]

Thanks for the ping @sdenef-adeo - will close this as it's obsolete.